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I've read a lot of comments over the years about Rickenbacker 4001 frets being soft and wearing rapidly if you use roundwounds etc. etc.

I played my own 4001 for more than 20 years (it was my only bass throughout that period) using various roundwounds, some of them stainless, before switching to flats a couple of years ago.
My frets are fine! .... and I'm fussy about that sort of thing!

Per John Hall they always used the hardest fret wire availabe.I was told not to use rounds by the local Ric dealer because of the truss rods which didn't make sense because flats have more tension than rounds. The 4003 had redesigned truss rods though. A lot of people cranked on the 4001 rods without "helping" push the neck in the desired direction. (Broke mine when I was 15 and was devistated until I found out they are replacable)
Always ran stainless Roto's without a problem.

If you wanna use steel strings then use them, I doubt you will have a problem unless you are really grinding the strings into the frets. If you are still worried about your frets, you can try some Alloy 52(nickel/iron) strings which can sound even nastier than a lot of steel rounds.

For a long time I believed the same thing. My 4003 turns 30 in May this year, and I guess the original owner never had the frets filed, because mine were quite shot when I got them. And keep steels on them for the couple subsequent years didn't help, I'm sure.

As an early birthday present to the bass, I just had the frets crowned by a friend who is a truly gifted guitar tech, and when he worked on them, he said they were the hardest frets he had ever tried to file. And he has literally filed hundreds of guitars in his lifetime! So if he says they were hard frets, I trust him. My bass is something of a mutt, because every discernible part is 70s old stock except for the TRC, and truss rod placement is 80s. I would therefore any 4001 of a reasonably similar vintage would have equally hard frets